1. Field of the Invention
The present invention relates to a burner particularly well adapted for use in a duct, such as a ventilation duct, for heating a stream of combustion supporting gas, like air.
The invention also relates to a process for heating a stream of combustion supporting gas.
2. Description of Related Art
"Direct-fired air heating" basically consists in using combustion products diluted in air for a heating application, such as the heating of a warehouse or some drying operation. Gaseous fuels which do not contain sulfur are particularly useful for this type of heating.
To proceed to such direct-fired air heating, a special type of burner has been developed, which is called "duct burner" because it consists of a piece of equipment that may be placed directly in a ventilation duct.
The conventional duct burners comprise a feed pipe with an integral baffle, called "bluff-body", which extends across a stream of air.
The feed pipe has a plurality of outlets for injecting a fuel gas in the direction of the stream. The first purpose of the bluff-body which is located upstream of the outlets, is to create a reverse flow in its center and a low velocity zone on both sides of the flame to stabilize the same. The second purpose of the bluff-body is to generate the turbulent energy required to ensure a complete mixing of the fuel gas with air. The problem with such a type of burner is that the stabilization of the flame and the mixing of the fuel gas are two opposing mechanisms. Increasing the size of the bluff-body in order to improve the flame stability results in a decrease in the mixing rate and vice versa.
The flame generated downstream of the bluff-body is intense, thus exposing the bluff-body to high temperatures. In order to sustain high temperatures, the bluff-body has to be made of heat resisting alloy blocks which substantially increases the cost of the burner. In order to minimize the costs, duct burner manufacturers have increased as much as possible the linear power of their burner by using very large bluff-bodies. The linear power is the amount of heat per unit length released by the burner. For example, a conventional duct burner releases around 5 MMBTU per hour per feet. Pursuing this trend of increasing the dimension of the bluff-body has brought major drawbacks. First of all, mixing capabilities are greatly diminished which results in a much longer flame that can be as long as 3 to 4 meters. A longer flame requires a longer combustion chamber which increases the overall cost of the application. Also, a reduced mixing rate can generate unacceptable levels of carbon monoxide emissions. Furthermore, increasing the dimension of the bluff-body also increases the pressure drop across the burner. The fan horsepower requirement increases as the dimension of the bluff-body increases. In addition, increasing the linear power tends to increase nitrogen oxide emissions.
In order to improve the duct burner's mixing rate, it appears imperative to reduce the size of the bluff-body while maintaining an efficient stabilization. A better mixing of the gaseous fuel with air allows for a better combustion and hence, a reduction of pollutant emissions.